1. Field of the Invention
The present invention relates to a seat for motor vehicles, and more particularly, to a link mechanism which governs the lifting and lowering portion in the height adjustment mechanism for the seat cushion.
2. Description of the Prior Art
In a seat for a motor vehicle, there is incorporated a height adjusting mechanism for the seat cushion. By the use of the mechanism it is possible to adjust the overall height of the seat cushion to match the seated height or the leg length of the user, and raise the rear end or lower the front end of the seat cushion. By so doing, the seat may be made to fit the user, permitting the user to put his feet at a proper position or secure a forward field of vision during driving.
A seat for a motor vehicle equipped with a height adjusting mechanism of the above kind includes an elevating mechanism of the seat cushion and an actuating mechanism for driving the elevating mechanism. As to the elevating mechanism of the above type, various techniques as disclosed in Japanese publications of unexamined utility model applications No. 57-194842, No. 58-49864, No. 60-90030 have been known. In addition, various kinds of techniques similar to the above have also been developed.
An elevating mechanism that is employed commonly in these techniques is mainly a parallel link mechanism. Namely, in these techniques, the upper parts of a pair of links that constitutes the parallel link mechanism are coupled with the front and rear of the seat cushion so as to have the height of the front section, rear section, or both of the seat cushion variably adjustable in response to the changes in the tilting angle of each link.
In a height adjusting means of the above kind, the rocking ends, namely, the upper portion, of the links are coupled directly with the seat cushion, and a control shaft is coupled with the turning center of the cushion so that the turning range of the control shaft is restricted.
In other words, in such a structure, the turning range of the control shaft is limited to the range of from 0.degree. to 90.degree. since the position of the maximum height corresponds to the state in which the links are upright and the position of the lowest height corresponds to the state in which the links are in the horizontal position. Accordingly, it is necessary to turn the control shaft in the reverse direction in order to readjust the seat cushion from either one of these states.
Further, as the operating system for the above kind of height adjustment mechanism, there are a manual type in which the control shaft is turned by the hand of the adjuster who grasps a knob which is provided on one end of the shaft, and a motor driven type, called power lift, in which the control shaft is linked via a reduction mechanism to the output shaft of a power motor which is rotated by the operation of a button, therefore power of the motor being transmitted to the link mechanism.
Accordingly, when the control shaft of the elevating mechanism that adopts the conventional parallel link mechanism is turned manually, there arises a possibility of applying an unreasonable force to the mechanism itself in an attempt to adjustment a range beyond the allowed limit of the operating range. On the other hand, it should be noted that the manual operation system may be advantageous to those who are accustomed to the system. This is because, due to the limitation on the turning of the control shaft, the operator is able to have a feel that suggests the limiting conditions of the rotation that is transmitted directly to his hand.
In contrast, the motor driven system requires a sensor and a device which can give the same kind of feel that the hand receives in the manual operation. These are a limit switch for detecting the stopping position of the motor rotation and a mechanism that carries out a control on the reverse rotation of the motor in response to the detected output of the switch. Consequently, the motor driven system becomes complicated in proportion to the added portion of the mechanism for controlling the rotation.
Therefore, it has to be concluded that neither the manual system nor the motor driven system is a desirable system in the sense that the necessity of regulating the range of rotation may adversely affect the rigidity of the mechanism itself or requires an addition of a mechanism for preventing such an undesirable effect.